The present invention relates to a free-standing wood-burning stove and particularly to a stove capable of exhibiting increased efficiency and economy of operation.
With the advent of increasing costs of fuels based on petroleum or natural gas, and the increasing possibility of the shortage of same, wood-burning stoves and heaters are enjoying a tremendous increase in popularity.
Conventional fireplaces, although attractive, are extremely inefficient and are not particularly adapted for use other than small area heating, and even that can be rather ineffective. Furthermore, conventional fireplaces typically cannot be conveniently controlled or regulated as to heat level or rate of combustion.
Free-standing or Franklin type stoves have the distinct advantage of greater heat utilization, while providing some of the attractiveness of the open fire of a fireplace. However, most stoves of this general type are rather inefficient in operation. For example, most of such stoves, regardless of claims to the contrary, produce visible smoke, which indicates inefficiency in operation.
Furthermore, the room heating possibilities of many of these stoves are not utilized other than simply via radiation to the surroundings, i.e., heating of ambient air only occurs through radiation from the walls of the stove.
Efficiency of stove operation relates to maximizing the completeness of combustion of the wood and volatile products, such as creosote, which result therefrom. Combustion is of course a chemical reaction releasing heat, with oxygen being the sole supporter necessary therefor. To maximize combustion, there must be found the precise proportion of oxygen required to combust the fuel, a thorough and complete mixing of the fuel and oxygen, and a sufficiently high temperature to promote rapid combination of oxygen with the existing fuel.
A measure of the efficiency of a stove is simply the effluent gas therefrom. Carbon, in the form of smoke, represents a serious inefficiency and waste of potential fuel. Similarly, the emission of carbon monoxide in the flue gas indicates inefficiency in operation.
There have been numerous approaches to increasing the efficiency of wood burning stoves in the prior art. In U.S. Pat. No. 4,182,304, there is disclosed a downdraft stove where the primary air for combustion is forced through the hot coals of the burned fuel. In U.S. Pat. No. 4,201,185, a thermostat system is placed within an air circulation chamber surrounding the combustion chamber which is connected to the primary combustion air draft control. In U.S. Pat. No. 4,232,653, primary combustion air is provided through a special orifice arrangement by a motor driven blower which is energized by a room thermostat.
All the foregoing provide attempts to optimize the efficiency of combustion when utilizing wood as the combustible fuel. However, we are presently not aware of a commercially available wood-burning stove which operates substantially without production of visible smoke.
Conversely, our wood burning stove is capable of burning substantially without visible effluent when the outside temperature is greater than about 20.degree. F., and during colder temperatures, in essence the only visible effluent is steam, signifying substantially perfect combustion.
In order to burn wood so that no visible smoke or creosote is produced, which is up to about 98 percent of the burn period, we have found that sufficient quantities of preheated air must be provided at differing levels within the combustion chamber. Furthermore, the lower level air must enter the primary combustion zone indirectly so that it does not flow or impinge directly onto the wood, thereby not igniting any more wood than the available air can combust. This lower level air feeds the front of the wood fuel with oxygen, and as the fuel burns backward, the heat produced by the reaction gradually induces the release of volatile liquids, which instantaneously gasify, from the remaining unburned portions of the wood. At the intermediate air level within the primary combustion zone, these gases combine with pre-heated air, and substantially complete combustion occurs. The balance of these unburned gases travel upward into a secondary combustion zone, where they pass through a layer of oxygen-rich pre-heated air, whereupon the combustion process is completed.
In our combustion chamber, with its primary and secondary combustion zones, the top, front, and sides are sufficiently insulated to insure that the reaction temperature therein remains at the necessary level for combustion to proceed. A temperature sensor at the entrance to the secondary combustion zone provides a signal to an automatic multi-stage control system for regulating the quantity of combustion air entering the primary combustion zone at the lower and intermediate levels.
Prior patents typically discuss "primary" or "secondary" air utilized in the operation of a stove, but we have found that the three levels of air in our stove act, at various stages of the combustion process, as both primary or secondary air. For example, when the primary combustion draft is open, some of the lower level air acts as both primary and secondary air, while the intermediate level air functions in the same manner. The secondary combustion zone air at this stage of the stove operation is strictly secondary in nature. However, when the primary combustion draft closes, and the stove begins to idle, the fire obtains its necessary combustion air (both primary and secondary) through a continuously open small draft damper, while the secondary combustion air is provided through continuously open draft dampers in the stove exterior. These dampers provide a steady flow of air into the secondary combustion zone at all times, thereby providing the necessary amount of air not available via the primary combustion draft, assuring virtually perfect combustion.
These features of using three distinct levels for providing combustion air, plus the concept of controlling the combustion chamber temperature by regulating the combustion air entering the primary combustion zone has been found to provide substantially smokeless stove operation.
In this fashion, our stove can burn wood containing a relatively high moisture content, at a controlled rate, with no visible effluent. In fact, if a filter paper is utilized to check the effluent of our stove, typically only traces of carbon will be found.